Field of the Invention
The invention concerns a method for magnetic resonance (MR) measurement (data acquisition) and a magnetic resonance system to implement such a method. In particular, the invention concerns techniques with which determination of first and second spectral components from MR data is possible.
Description of the Prior Art
In a magnetic resonance measurements data acquisition, it is possible to separate spectral components included in the MR data. The spectral components can represent different spin species, for example nuclear spins in a fat environment and in an aqueous environment. For this purpose, chemical shift imaging multi-echo magnetic resonance (MR) measurement sequences are often used within the context of Dixon techniques. Such techniques typically utilize the fact that the resonance frequency of nuclear spins depends on the molecular or, respectively, chemical environment. This effect is known as a chemical shift. Different spin species have different resonance frequencies, from which the measured spectrum of the MR data is composed. For example, the difference between two resonance frequencies of different spectral components can be expressed in ppm (parts per million).
The chemical shift between hydrogen nuclear spins in water as a first spectral component, and hydrogen nuclear spins in fatty acid chains as a second spectral component, is often used. In such a case, a water MR image and/or a fat MR image —i.e. individual MR images of the two spectral components—can be produced using MR data. This is of interest in a variety of applications, for example clinical and/or medical applications.
In order to be able to separate the spectral components from one another, MR signals are acquired at multiple echo times within the scope of the Dixon technique. The MR signals together form the MR data. The different spectral components have different phase positions at the different echo times. Using this effect, it is possible to determine the different spectral components separately.
For this purpose, a spectral model is generally used that links the measured or acquired MR data with different physically relevant variables. The different variables in particular include the different spectral components to be determined, as well as additional unknowns of the measurement system (depending on the precision, scope and complexity of the spectral model). It can then be possible to determine the spectral components considered in the spectral model for each image point of the MR data.
In principle, it can be worthwhile to use a relatively complex spectral model, for example such a spectral model which considers a large number of further unknowns in addition to the spectral components to be determined. It can then be possible to determine the spectral components particularly precisely. In this case, however, it can be necessary to acquire particularly many MR signals at different echo times, which can in turn extend a measurement duration and therefore can be disadvantageous. A trade-off situation thus often results between measurement duration and precision in the determination of the spectral components.